The invention relates to an apparatus for developing an electrostatic latent image formed on an image bearing member such as photosensitive member or record member, and in particular to such apparatus which utilizes one-component developer without requiring the use of a carrier.
A developing apparatus using one-component developer is known, see, for example, U.S. Pat. Nos. 3,093,039 and 3,909,258 which may utilize a magnetic toner of either high or low resistivity. When magnetic toner of low resistivity is used, the low resistivity prevents charging of the toner, so that the toner is attracted to the electrostatic image by electrostatic induction. Because the toner itself is not charged, it is inadequate for use with an image of low potential. In general, the use of a magnetic toner facilitates transportation of the toner since it may be magnetically attracted to the surface of a developing roller under the influence of a magnet, but requires, for the fixing of the developed toner image, an increased amount of heat as high as 1.5 to 2 times the heat quantity required for normal toner particles alone as a result of the presence of a magnetic powder, such as iron powder which has a relatively high heat capacity in admixture with the toner. This presents difficulties with respect to operational safety and high speed copying operations. With a non-magnetic toner, the toner can be charged to assure full developing of an image of low potential. However, since the toner has no magnetic property, magnetic attraction cannot be utilized for the conveyance thereof, and the absence of a stirring action under the magnetic influence tends to cause agglomeration of the toner, resulting in a nonuniformity in the charging and developing effects. While both magnetic and non-magnetic toners as one-component developers have certain drawbacks and advantages, they do not require the use of a carrier, thus dispensing with the need to control the density of the developer, and also facilitating the handling of the developer. As a consequence, the present trend is toward the use of one-component developer in preference to two-component developer.
There remains the problem of determining whether the magnetic or non-magnetic toner is more advantageous for use as one-component developer. While a general conclusion cannot be drawn since they are advantageous and disadvantageous in respective different aspects, it can be seen that a toner of low resistivity does not lend itself to the developing of an electrostatic image having an increased proportion of thin lines without some special provision, as a result of the inability of charging such toner.